# ch15notes - CHAPTER 15 THE SOLAR SYSTEM The solar system...

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CHAPTER 15 - THE SOLAR SYSTEM The solar system consists of the sun, nine planets and their 70 or so moons, asteroids, comets, meteoroids, interplanetary dust particles, gasses and a solar wind composed of charged particles. Early studies of the solar system incorrectly assumed that the earth was stationary and everything else revolved around the earth. This was the Ptolemy version of the solar system and was called the geocentric (earth-centered)model. In the early 1500's, Copernicus developed the idea that the sun was the center of the solar system and that the earth revolved around it. Brahe was a Danish astronomer who made the most accurate measurements at that time of positions of the planets and stars. He did this without a telescope since it had not been invented yet. Kepler was a German mathematician and astronomer who analyzed the data Brahe collected to develop three laws that govern the motion of planets in the solar system. They are collectively called Kepler's laws of planetary motion. Kepler's first law states that all planets move in elliptical orbits around the sun with the sun at one focus of the ellipse. An ellipse is shaped like a circle that has been flattened so that it is not perfectly round. The longest distance across an ellipse is called its major axis and half of this distance is called its semi major axis. The radius of a planet's orbit is approximately equal to its semi major axis.

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Distances in the solar system are often expressed in terms of Astronomical Units. 1 AU equals the length of the earth's semi major axis (average distance from the earth to the sun). Kepler's second law is the law of equal areas. It states that a radius vector drawn from any single planet to the sun will sweep out equal areas in equal times. This is a result of the conservation of energy. When the planet is far away from the sun, the radius vector forms a long, skinny pie slice because it is moving more slowly(less kinetic, more potential energy). When the planet is close to the sun, the radius vector forms a short, fat pie slice because it is moving faster(more kinetic, less potential energy). Kepler's third law is known as the harmonic law because of the name of the paper he wrote to describe it (Harmony of the Worlds). It states that the square of the sidereal period of a planet is proportional to the cube of its semi major axis. The equation is: T 2 = kR 3 In the equation, T is the sidereal period of the planet (time measured with respect to a distant star), R is the length of the semi major axis of the orbit and k is a proportionality constant. If we express the period in earth years and the radius of the orbit in AU's, k = 1 year 2 /AU 3 Example The distance from Mars to the sun averages 1.52 AU. Find the time in earth years required for Mars to complete one orbit.
Another very important scientist/astronomer was Galileo. He was the first person to observe the moon and planets through a telescope. He discovered four of Jupiter’s moons which showed

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ch15notes - CHAPTER 15 THE SOLAR SYSTEM The solar system...

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